Darkness/lightness control is a feature commonly known to be employed in xerographic reproduction systems. Specifically, it is well-known to control the darkness/lightness of an output document by altering the xerographic processes employed to generate and deposit an image on a substrate. However, such changes to the xerographic process were often imperfectly applied over an entire document, leading to user dissatisfaction, or were particularly problematic for the xerographic system, often resulting in reduced life of the xerographic components and supplies.
The present invention is a method and apparatus for accomplishing darkness/lightness control in a digital printing system, wherein the control is accomplished by directly controlling the image exposure system rather than by indirectly controlling the amount of marking material deposited upon an image substrate. The present invention takes advantage of the improved reliability and resolution of digital image printing systems to accomplish the real-time processing of digital image data so as to alter the data in accordance with a user-specified darkness/lightness level in a document image.
Heretofore, a number of patents and publications have disclosed bitmap image processing methods, the relevant portions of which are hereby incorporated by reference:
U.S. Pat. Nos. 4,460,909, 4,544,264, and 4,625,222 to Bassetti; 5,091,971 to Ward et al. issued Feb. 25, 1992; 5,282,057 to Mailloux et al., issued Jan. 25, 1994; 5,359,423 to Loce, issued Oct. 25, 1994; 5,387,985 to Loce et al., issued Feb. 7, 1995; 5,483,351 to Mailloux et al., issued Jan. 9, 1996; and 5,555,557 to Mailloux, issued Sep. 10, 1996.
U.S. Pat. No. 5,128,698 to Crawford et al., issued Jul. 7, 1992, for Boldness Control in an Electrophotographic Machine, employs control of the illumination intensity at edge pixels to control placement of image edges.
The book "Enhancement and Restoration of Digital Documents: Statistical design of nonlinear algorithms" by R. Loce & E. Dougherty, SPIE Optical Engineering Press (1997) also describes template-based morphological operations as applied to image bitmaps. The relevant portions of the publication are hereby incorporated by reference.
Crawford, J. L. and C. D. Elzinga, "Improved Output Quality by Modulating Recording Power," SPSE 41st Annual Conference, May 22-26, 1988 Arlington Va., describes logic mask techniques to smooth bit map images while dilating image structures. Power modulation of the laser is employed.
In accordance with the present invention, there is provided a method of altering the darkness level of a digital image, comprising the steps of: receiving a bitmap array of image signals; isolating, within the array of image signals, image structures to be altered and storing the image structure to be altered; from the stored image structures, determining the border regions thereof to be altered; altering the border region of the stored image structure so as to change the output signal of at least one pixel along the border region; and applying the altered, stored image structure to a corresponding region in the image bitmap to alter the level of darkness of a region therein.
In accordance with another aspect of the present invention, there is provided a method of altering the level of darkness of an image in conjunction with increasing the resolution thereof prior to rendering the image with a marking engine, comprising the steps of: retrieving a requested darkness level; receiving an array of image signals in the form of an image bitmap at a resolution N; selecting, from the array of image signals, a window of image signals for processing, the window being centered about a target pixel; selecting, in response to the requested darkness level, one of a plurality of sets of template patterns, each set of template patterns containing a plurality of templates therein; comparing the window of image signals to each template within the selected one of the plurality of sets of template patterns to identify a match therewith; in the event of an affirmative response to the comparing step generating an output signal at a resolution M, where M is an integer multiple of N and is determined by the template matched, otherwise generating an output signal at a resolution M wherein each bit of the output signal is equivalent to the input signal level of the target pixel.
In accordance with yet another aspect of the present invention, there is provided an apparatus for altering the level of darkness of an image, including: means for setting a requested darkness level; image memory for storing an array of image signals in the form of an image bitmap at a resolution N; windowing circuitry for selecting, from the array of image signals, a region of image signals for processing, the region being centered about a target pixel; means for preprogramming each of a plurality of sets of template patterns, each set of template patterns containing a plurality of templates and each set of template patterns being associated with a unique darkness level; and matching circuitry for comparing the window of image signals to each template with a selected one of the plurality of sets of template patterns to identify a match therewith, wherein the selected one of the plurality of sets of template patterns is determined in response to the requested darkness level, said comparing circuitry, in the event of a match, further generating an output signal at a resolution M, where M is an integer multiple of N and is determined by the template matched, otherwise said comparing circuitry generating an output signal at a resolution M wherein each bit of the output signal is equivalent to the input signal level of the target pixel.
One aspect of the invention deals with a basic problem in printing and reproduction of images--modifying bitmap images so as to alter the perceived darkness or lightness in a digital image printing system. It has been noted that it is possible to alter the xerographic engine settings in order to accomplish such controls in a conventional manner. However, the ability to process bitmap images in real-time lends itself to the control of the darkness/lightness of the output via modification of the bitmap. Unfortunately, such controls are typically applied uniformly across an entire document, resulting in modifications to regions of a bitmap where alterations are undesirable and leading to visually displeasing output. This aspect is further based on the discovery of a technique that alleviates the need for conventional darkness lightness control while avoiding problems associated with uniform application of bitmap erosion and dilation. The technique preferably employs templates to selectively apply different amounts of darkening (lightening) to areas dependent upon the need for darkening (lightening). An added advantage of accomplishing the darkness/lightness control using this technique is that there is little variation in the xerographic controls and, therefore, improved system life and image quality.